TypedArray.from() has the optional parameter mapFn, which allows you to execute a map() function on each element of the typed array (or subclass object) that is being created. This means that the following are equivalent:

When the source parameter is an iterator, the TypedArray.from() first collects all the values from the iterator, then creates an instance of thisArg using the count, then sets the values on the instance. Array.from() sets each value as it receives them from the iterator, then sets its length at the end.

When Array.from() gets an array-like which isn't an iterator, it respects holes. TypedArray.from() will ensure the result is dense.

Polyfill

You can partially work around this by inserting the following code at the beginning of your scripts, allowing use of much of the functionality of from() in implementations that do not natively support it.